Hydrazine derivatives which are used in industry and medicine are known carcinogens. Hydralazine, a potent antihypertensive drug is also carcinogenic. However, the mechanism of carcinogenicity is not clearly understood. The oxidative metabolism of hydralazine has been studied by means of electron spin resonance spectroscopy and spin trapping. A nitrogen-centered hydralazyl radical was detected in the presence of metal ions or red blood cells and was also formed enzymatically. This radical or oxygen derived species may play a role in the toxicity of this drug.